An embolus can be any particle comprising a foreign or native material, which enters the vascular system or other vessel of the body with potential to cause occlusion of blood flow. Emboli can be formed from aggregates of fibrin, blood cells or fragments thereof, collagen, cholesterol, plaque, fat, calcified plaque, bubbles, arterial tissue, and/or other miscellaneous fragments. Most emboli lodge in the narrowing regions of medium size blood vessels that feed the major organs. Loss of blood flow to surrounding tissue causes localized cell death or microinfarcts. Cerebral microinfarcts can cause stroke leading to confusion, disturbance of speech, paralysis, visual disturbances, balance disturbances and even death (5). In the heart, emboli can cause myocardial infarcts, i.e. heart attacks.
Disease states including arteriosclerosis and deep vein thrombosis, aging and even pregnancy can cause build up of plaque and fibrin on vessel walls. Anything which loosens or breaks up this plaque can generate emboli. The clinical ramifications of emboli are staggering. Emboli generated from arteriosclerosis of the carotid artery alone cause 25% of the 500,000 strokes that occur yearly in the United States (2002 American Heart Association And Stroke annual statistics). Ironically, the surgical interventions used to remove or bypass the plaque of arteriosclerosis (e.g., balloon dilatation angioplasty, endarterectomy, bypass grafting and stenting) can themselves disrupt plaque. One of the most common cardiovascular interventions is coronary artery bypass grafting (CABG). Historically, 10-20% of all CABG interventions generate emboli large enough to cause myocardial infarcts. This is particularly true when the graft used is of saphenous vein origin. But CABG is not the only procedure with potential to generate emboli. In fact, doppler ultrasound shows evidence of microembolization in almost all cardiac intervention cases. Of the over 1.8 million intervention procedures performed annually, greater than 10% result in neurocognitive disturbance and/or ischemic event (8). These impairments are frequently short term, but can be permanent.
Ten percent is currently considered an acceptable complication rate, however as the number of procedures continues to grow (15-35% increase annually depending on specific procedure (Medical And Healthcare Marketplace Guide, 17th Edition Volume 1, Research Reports 2001-2002.)) the total number of patients affected grows. As this number increases so does patient care spending. Approximately $2.5 billion is spent annually on patients undergoing percutaneous endovascular procedures. The average stay for patients without adverse embolic outcomes is 8.6 days, while patients with severe adverse outcomes stay an average of 55.8 days (1, 3). Estimating the average hospital day care cost at $1500/day, extended stays due to embolic events cost greater than $60,000 per patient. While daunting, this figure fails to include the social and financial burden placed on family members upon hospital release. In summary, embolic events complicating percutanuous endovascular procedures cause high rates of clinically observed neurological disturbances and cardiovascular disease, decreased quality of life and increased patient care spending. Thus, there is a significant clinical need for effective prevention of adverse embolic events.
A significant reason for ischemic injury during percutaneous procedures can be generation of emboli which block smaller distal vessels. One approach to curb this complication has been to use pharmacological therapies during the time of the intervention. Limited therapeutic success has been reported with the use of calcium channel blockers, adenosine, and sodium nitroprusside (Webb, J G, Carere, R G, Virmani, R, Baim, D, Teirstein, P S, Whitlow, P, McQueen, C, Kolodgie, F D, Buller, E, Dodek, A, Mancini, G B, & Oesterle, S: Retrieval and analysis of particulate debris after saphenous vein graft intervention. J Am Coll Cardiol 2000, 34:468-475.). Glyoprotein IIb/IIIa inhibitors have been used for percutaneous coronary interventions to reduce platelet aggregation, but also fail to show meaningful long term clinical benefit. (Mathew, V, Grill, D E, Scott, C G, Grantham, J A, Ting, H H, Garratt, K N, & Holmes, D R, Jr. The influence of abciximab use on clinical outcome after aortocoronary vein graft interventions. J Am Coll Cardiol 1999, 34:1163-1169 and Mak, K H, Challapalli, R, Eisenberg, M J, Anderson, K M, Califf, R M, & Topol, E J: Effect of platelet glycoprotein IIb/IIIa receptor inhibition on distal embolization during percutaneous revascularization of aortocoronary saphenous vein grafts. EPIC Investigators. Evaluation of IIb/IIIa platelet receptor antagonist 7E3 in Preventing Ischemic Complications. Am J Cardiol 1997, 80:985-988.) Lack of benefit may be attributed to the way in which these emboli are generated. In most cases embolization develops from physical disruption of fibrotic plaque. Thus the mechanism of therapeutic embolic protection specifically targeted at prevention of platelet aggregation and blood clotting has little effect on these already-formed, embolizable plaques.
Cardiac Surgery
Each year there are approximately 800,000 cardiac surgical cases, which involve cardiopulmonary bypass (CPB) worldwide. Of these cardiac surgical cases, approximately 48,000 suffer stroke and nearly 300,000 experience some neurocognitive disturbance. This is a significant clinical problem. These complications are due in large measure to CPB-generated emboli. The average number of emboli measured by Trans Cranial Doppler (TCD) in patients undergoing cardiopulmonary bypass is 183 (range 3-947). The majority of emboli end up in the very distal cerebral tree, the terminal arterioles and capillaries causing microinfarctions, (i.e., loss of blood to surrounding tissue). Pathological evaluation of affected tissues reveals sausage-shaped arterial dilatations known as SCADs. Cerebral microinfarctions can cause confusion, disturbances of speech, paralysis, visual disturbances, balance disturbances and other neurological deviations. These impairments are frequently short term but can be permanent.
Cardiology and Endovascular Intervention
Many clinical procedures can result in emboli including, for example, coronary, carotid, and peripheral interventions. (8) In these cases, particulate matter, including, for example, plaque, debris and thrombus, can form emboli distal to the site of intervention. As a result, blood flow to the distal vascular bed can be diminished and periprocedural end-organ ischemia and infarction can result. Distal embolization of large particles produced at the time of such interventions as balloon inflation or stent deployment may obstruct large, epicardial vessels, and smaller particles (as small as 15-100 microns) can cause microinfarcts and/or myocardial infarctions and left ventricular dysfunction. (8) Myocardial infarction refers to the death of a section of myocardium or middle layer of the heart muscle. Myocardial infarction can result from at least partial blockage of the coronary artery or its branches. Blockage of capillaries associated with the coronary arteries can result in corresponding microinfarctions/microinfarcs.
Renal Interventions
Surgical procedures for the treatment of renal artery stenosis can also generate emboli. There is clinical evidence to suggest that 36% of those treated suffer arterioloar nephrosclerosis caused by atheroemboli. Five-year survival of patients with atheroembolic events is significantly worse than of patients without atheroemboli (54% vs. 85% respectively) [Krishmamurthi et al. J Urol. 1999, 161:1093-6]. These patients could also benefit from distal protection devices.
Emboli and Infection
Foreign material in the stream of flow can cause turbulence or low flow. Such flow conditions have been shown to increase rates of infection. Thrombus not only generates emboli, but also increases the risk of infection. (9)
It is evident that a wide variety of embolic events cause high rates of clinically observed symptoms, decreased quality of life and increased patient care spending.